Thimerosal [54-64-8] Nomination to the National Toxicology Program Review of the Literature April 2001 1 Executive Summary The nomination of thimerosal is based on its wide use as a preservative in vaccines and other biological products, the large number of exposures, and the lack of toxicity data. Thimerosal (sodium ethylmercurithiosalicylate; also called thiomersal and merthiolate) was developed by Eli Lilly in the 1930s and has been used as a preservative in vaccines and other products because of its bacteriostatic and fungistatic properities. It is prepared by the interaction of ethylmercuric chloride or hydroxide with thiosalicylic acid and sodium hydroxide, in ethanol. Human exposure to thimerosal occurs through use of biological products such as certain vaccines, antivenins and immune globulin preparations, as well as some drug products including ophthalmic, otic, nasal and topical products. A review by the FDA in 1999 estimated that thimerosal was used in over 30 licensed and marketed vaccines and biologics. In recent years the largest exposure to thimerosal in terms of numbers exposed and amount (µg/kg body weight) may have been through vaccinations. Every year, approximately 4 million infants (the U.S. birth cohort) receive vaccines according to the U.S. routine childhood immunizations schedule. During the past decade, additional vaccinations have been added to the routine childhood immunization schedule, and until recently, some of these vaccines contained thimerosal as a preservative. Prior to the recent approval of additional thimerosal-free or thimerosal-reduced vaccines, an infant may have received a total mercury dose from vaccines as much as 187.5 µg during the first 6 months of life. In special populations, influenza vaccine may be administered at 6 months of age, which would increase the total dose to approximately 200 µg. Although thimerosal has been used in the U.S. as a preservative in vaccines and other licensed products since the 1930s, limited data are available on the toxicology of thimerosal and its metabolite ethylmercury. In humans, the only well-established hazard of thimerosal at doses found in vaccines is delayed-type hypersensitivity reactions. At very high doses, the identified hazards of thimerosal include neurotoxicity and nephrotoxicity. Only one published study evaluated the effect of thimerosal in vaccines on blood levels of mercury. This study measured the total mercury blood levels before and after hepatitis B vaccination in a small number of term and preterm newborn infants and suggested that a birth dose of hepatitis B vaccine may produce small but measurable increases in blood levels of mercury. In order to assess the potential health effects of exposure to thimerosal in childhood vaccines, the Centers for Disease Control and Prevention (CDC) sought epidemiological data to examine selected outcomes with varying exposure levels of thimerosal. This “screening analysis” found weak (relative risk less than 2) but statistically-significant associations between exposure to thimerosal-containing vaccines before the age of 6 months and tic disorders, attention deficit disorders (ADD), and speech and language disorders. The investigators then used another, smaller database from the East Coast for a more focused study to test the hypotheses that tic disorders, ADD, and speech and language disorders are associated with thimerosal exposure before 6 months of age. This study did not confirm an association. Taken together, the results of the two studies are inconclusive as to an effect of thimerosal on neurological outcomes. 2 Only limited data were available on the reproductive and teratogenic effects of thimerosal. In one study of pregnant rats and rabbits receiving intraperitoneal injections and ocular instillations, no teratological effects or evidence of maternal toxicity were observed, but dose related embryo­ and fetal lethality was found. A comparison of topical and subcutaneous administration of thimerosal to rabbits showed measurable mercury present in blood and tissues of the treated animals and their offspring, although no sign of tissue damage was apparent by light microscopy. Thimerosal was found to cross the blood-brain and placenta barriers. Limited information on the carcinogenicity and genetic toxicity of thimerosal was found. In a toxicology and carcinogenesis study, rats were subcutaneously injected twice-weekly with thimerosal at doses ranging from 30 to 1000 µg /kg for 1 year. Histological observations included findings of lung tumors at a similar incidence to negative controls or at lower incidence than positive controls. Thimerosal-injected animals demonstrated a dose-related inhibition of spontaneous interstitial cell tumors of the testicles. In a test of genetic toxicity, thimerosal was not found to be mutagenic in Salmonella typhimurium. In vivo, thimerosal did not induce aneuploidy. Methylmercury, an organic mercurial similar to ethylmercury, has been associated in some studies with subtle neurodevelopmental abnormalities at low doses. There exists an extensive body of research on the toxicity of methylmercury, but the applicability of these data to the toxicity of ethylmercury are not currently known. Limited data were found on the comparative toxicology of ethylmercury vs. methylmercury. One animal study directly compared the toxicity of these compounds in rats administered 5 daily doses (8.0 or 9.6 mg/kg) of equimolar concentrations of ethyl- or methylmercury by gavage. Tissue distribution, and the extent and severity of histological changes in the brain and kidney were assessed. Neurotoxicity of ethyl­ and methylmercury was similar, with higher levels of inorganic mercury observed in the brains of ethylmercury treated rats. Renal damage was greater in rats receiving ethylmercury. Although the data are limited, similar toxicological profiles between ethylmercury and methylmercury raise the possibility that neurotoxicity may also occur at low doses of thimerosal. Under the FDA Modernization Act of 1997 requiring the study of the “adverse effects on health of children and other sensitive populations from exposure to … mercury", FDA conducted a review of the use of thimerosal in childhood vaccines. FDA compared exposure levels of infants to ethylmercury from vaccines to existing guidelines for exposure to methylmercury, as there are no existing guidelines for safe exposure to ethylmercury, the metabolite of thimerosal. While this review found no evidence of adverse effects caused by thimerosal in vaccines, except for minor local hypersensitivity reactions, the assessment determined that the use of thimerosal as a preservative in vaccines might result in the intake of mercury during the first six months of life that exceeded recommended guidelines from the Environmental Protection Agency (EPA), but not guidelines from the Agency for Toxic Substance and Disease Registry (ATSDR), FDA, or World Health Organization (WHO). As a precautionary measure, in July 1999, the Public Health Service (PHS), along with the American Academy of Pediatrics and the American Academy of Family Physicians, issued a joint statement on thimerosal and vaccines asking manufacturers to reduce or eliminate 3 thimerosal as a preservative in childhood vaccines and substantial progress has been made to date. With the recent approval of a new formulation of one of the licensed diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccines, all routinely recommended pediatric vaccines in distribution will contain no thimerosal or markedly reduced amounts. There is continued interest, however, on the part of the public as well as PHS agencies to better characterize the potential toxicity of thimerosal. In the U.S., thimerosal is still present as preservative in some vaccines given to young children, as well as certain biological products recommended during pregnancy. Thimerosal remains a preservative in some vaccines administered to adolescents and adults. In addition, thimerosal continues to be used internationally as a vaccine preservative. Further data are needed to determine whether harmful effects have occurred from prior exposure to thimerosal or from its continued use as a preservative in the U.S and international settings. Thimerosal is nominated to the NTP for further study to assess gaps in knowledge regarding toxicokinetics and the potential for neurodevelopmental toxicity. These gaps include comparative toxicity of ethyl- and methylmercury, the metabolism and elimination of ethylmercury compared with methylmercury, the effect of intermittent intramuscular doses of thimerosal from vaccines compared with chronic low dose oral exposure to methylmercury, and the susceptibility of the infant compared with the fetus to adverse effects from organic mercurials. In order to provide a more complete assessment of the toxicity of thimerosal during the critical period of neurodevelopment, well-designed studies are needed to address these gaps in knowledge in appropriate animal model(s). 4 Table of Contents 1.0 Basis of Nomination ........................................................................................................ 6 2.0 Chemical Properties........................................................................................................7 2.1 Chemical Identification ....................................................................................... 8 2.2 Physical-Chemical Properties ............................................................................. 8 2.3